Chemical vapour deposition (CVD) is used to deposit thin films or layers on the surface of a substrate or wafer located in a deposition chamber. This process operates by supplying one or more reactive gases, often using a carrier gas, to the substrate's surface under conditions that encourage chemical reactions to take place at the surface. For example, TEOS and one of oxygen and ozone may be supplied to the deposition chamber for the formation of a silicon oxide layer on the substrate, and silane and ammonia may be supplied for the formation of a silicon nitride layer.
The production of silicon thin film solar cells requires the sequential deposition of p-type, i-type and n-type layers of silicon on a glass substrate having a transparent conductive layer formed thereon. Each silicon deposition is conducted in a respective chamber in which a plasma-enhanced CVD method is performed by the application of a high frequency power to an electrode located within the chamber. For example, the glass substrate is initially located in a first evacuated chamber into which a gas mixture containing silane (SiH4), a dopant such as B2H6 and hydrogen (H2) as a carrier gas is supplied to form a p-type silicon layer on the substrate. The substrate is then moved to a second evacuated chamber, into which a gas mixture of H2 and SiH4 is supplied to form an i-type silicon layer on the substrate. The substrate is then moved to a third evacuated chamber, into which a gas mixture of H2, SiH4 and a dopant such as PH3 is supplied to form an n-type silicon layer on the substrate. Depending on the nature of the solar cell, the substrate may be moved between further chambers for the deposition of further silicon layers on the substrate. Upon completion of the silicon deposition, the substrate is moved to a final chamber in which a second transparent conductive layer and an electrode are formed on the substrate, for example using a sputtering technique, to complete manufacture of the solar cell.
There is a trend in the manufacture of devices such as solar cells to perform deposition on increasingly larger substrates to deliver economies of scale, with the substrate being diced upon completion of the deposition steps to produce a multiplicity of individual devices of the required size. As a result, the size of the deposition chambers and the flow rates of the gases supplied thereto, in particular that of carrier gases such as H2, must also increase to accommodate the larger substrates and produce acceptable deposition rates. For example, currently the flow rate of H2 into a chamber of a solar cell manufacturing tool is around several hundred slm, but it is envisaged that future generation tools will require a H2 flow rate of at least 1000 slm into each chamber.
As the flow rate of gas entering a deposition chamber increases the size of the vacuum pumping system used to evacuate the chamber and draw the unconsumed process gases and any reaction by-products from the chamber must also increase. Furthermore, the gas stream drawn from the chamber requires treatment before it is exhausted into the atmosphere to remove any potentially hazardous gases such as H2. For this reason, the gas stream is usually conveyed to an abatement device in which the hydrogen is burnt in a controlled manner. The size and energy consumption of a pumping and abatement system used to pump a gas stream containing 1000 slm of H2 to atmospheric pressure and then treat a gas stream, together with the associated capital cost of such a system and the cooling water consumption of the abatement device, will be significant. There are also safety issues associated with large flows of hydrogen at atmospheric pressure. Whilst H2 is a relatively inexpensive gas, there will be costs associated with the transportation of large amounts of hydrogen to the process tool.
Consequently, a more cost-effective technique for handling a gas stream containing a relatively large amount of H2 is required.